Abstract

We report what is to our knowledge the first observation of the effect of parallel-to-interface-refraction (PIR) in a three-dimensional, simple-cubic photonic-crystal. PIR is an acutely negative refraction of light inside a photonic-crystal, leading to light-bending by nearly 90 deg over broad wavelengths (λ). The consequence is a longer path length of light in the medium and an improved light absorption beyond the Lambertian limit. As an illustration of the effect, we show near-unity total absorption (≥98%) in λ=520–620nm and an average absorption of ∼94% over λ=400–700nm for our α-Si:H photonic-crystal sample of an equivalent bulk thickness of t˜=450nm. Furthermore, we have achieved an ultra-wide angular acceptance of light over θ=0°–80°. This demonstration opens up a new door for light trapping and near-unity solar absorption over broad λs and wide angles.

Figures (4)

Anomalous light refraction in a 3D photonic-crystal. (a) Schematic of a simple-cubic lattice structure. The direction of light refraction is indicated as red arrows. (b) SEM image of the fabricated sample with a lattice constant of a=350nm. (c) The corresponding first Brillouin zone in k-space. (d) The calculated dispersion, frequency versus wave vector, of the lattice. The green square box corresponds to wavelengths of λ≈450–800nm. (e) An iso-frequency plot of band-3. It has a square-like contour, allowing light refraction PIR. The red arrows indicate the flow of energy. (f) An iso-frequency plot of band-27, which is more complicated but still exhibits a square-like contour.

Measured and computed total transmission, reflection, and absorption spectra. (a) The measured spectra for a 600 nm thick α-Si:H thin film, showing an absorption of A≈60% in 400nm≤λ≤580nm. In the gray region, the data shows a weak absorption (A≤30%) and also resonant oscillations in both the transmission and reflection spectra. (b) The a=350nm photonic-crystal sample shows a higher absorption of A≈85% in 400nm≤λ≤630nm. (c) The a=450nm photonic-crystal shows an even higher absorption of A∼90% in 400nm≤λ≤640nm.

Enhancement factor and broadband absorption. (a) Enhancement of absorption of the 4L simple-cubic samples over the 600 nm α-Si:H thin film. The dotted line represents the maximum enhancement needed to reach 100% absorption. (b) Total absorption/(1-R) plot, showing that the absorption edge (the vertical arrows) is systematically shifted to longer λs as the lattice constant is increased. The light-gray and dark-gray dotted lines are the Lambertian limits, calculated using our measured optical constants and those taken from the literature, respectively. (c) Plot of optical constants measured from α-Si:H thin film (light-gray dots) and those taken from the literature (dark-gray dots).